Various electronic devices such as klystrons, traveling wave tubes and other microwave devices require controlled uniform magnetic fields. The advent of rare earth-cobalt permanent magnets makes it possible to create many novel magnetic structures that were formally not practicable. The desirable properties of rare earth-cobalt permanent magnets stem from their high saturation magnetization and high coercivity. A high saturation magnetization provides a magnetic field source of high flux density; a high coercivity enables the magnet to maintain its flux density in the face of very high demagnetizing fields. Thus, rare earth-cobalt permanent magnets can with impunity be fashioned in shapes that would cause demagnetization of conventional magnetic materials such as alnico.
Analysis of magnetic circuits is often facilitated by comparing them with well-known dc electrical circuits. In this scheme, the roles of electric current I, electric force V, electric conductance G, and resistance R, are assumed respectively, by the magnetic flux .phi., the magnetomotive force F, the magnetic permeance P, and the reluctance . In this analogy, permanent magnets may be viewed as "magnetic batteries"; materials of high permeability, such as soft iron or permalloy may be viewed as essentially perfect flux conductors; and air gaps or materials of low permeability as "magnetic resistors". Unlike alnico magnets, rare earth-cobalt magnets are well suited to the above analogy, because they generate a magnetomotive force which is independent of the circuit into which they are inserted (similar to a battery). By contrast, the magnetomotive force provided by an alnico magnet is affected by the particular magnetic circuit into which it is inserted.
Those concerned with the development of magnetic devices have long recognized the need for improving the magnetic flux density per unit weight of magnetic circuits, thereby improving the overall size and cost of such devices. Various prior art devices have used magnetic cladding to reduce flux leakage exterior to the circuit and increase the desired controlled magnetic field intensity in the working space interior to the circuit without appreciably increasing the total size or weight of the magnetic circuit. In accord with the electrical analogy mentioned above, reduction of current in a branch of an electrical circuit may be effected either by increasing the resistance in that branch or by inserting a source of electromotive force in opposition to the unwanted current. The use of magnetic cladding in magnetic circuits corresponds to setting up a magnetomotive force to counter undesired flux flow external to the circuit working space. The cladding magnets provide a "bucking" magnetomotive force which prevents flux "flow" (akin to current flow) where it is not wanted by the designer. In other words, flux is confined (or prevented from leaking out) by making the entire outer surface of a magnetic circuit an equipotential surface so that no flux lines can stream between different points on it.
Two magnetic structures of particular importance to this invention are the magic ring and a cladded rectangular structure. One embodiment of the magic ring is disclosed in: K. Halbach, proceedings of the 8th International Conference on Rare Earth Magnetic Materials (University of Dayton, Dayton Ohio, 1985), p. 123. The cladded rectangular structure is disclosed in: applicant's copending U.S. patent application entitled "Permanent Magnet Structure for a Nuclear Magnetic Resonance Imager for Medical Diagnostics, Ser. No. 112,192 filed Oct. 20, 1987 which is hereby incorporated by reference.
The ideal magic ring is an infinitely long, annular cylindrical shell which produces an intense magnetic field in its interior working space. The direction of the magnetic field in the working space interior is perpendicular to the long axis of the cylinder. The Halbach publication discloses a structure with an octagonal cross section which closely approximates the performance and field configuration of an ideal magic ring. In both the ideal and Halbach configurations, no magnetic flux extends to the exterior of the ring structure.
The cladded rectangular structure of the above-mentioned copending application is also (theoretically) infinitely long in one dimension. The structure produces a strong uniform magnetic field within a rectangular working space. The direction of the magnetic field is perpendicular to the long dimension of the structure. The structure has primary external cladding magnets which prevent flux from escaping to the exterior of the structure.
Unfortunately, both the magic ring and cladded rectangular structure of the above-mentioned copending application are theoretically infinitely long. Thus, achievement of the desirable high uniform magnetic fields in the interiors of both of these structures demands that both structures be made extremely long (theoretically infinite). If either structure is not long enough, distortion of the interior fields will result.
Those concerned with the development of magnetic devices have long recognized the need for improving the magnetic density per unit weight of magnetic circuits and thereby improving the overall size and cost of such devices. Thus, there is a need to fabricate both a magic ring and a cladded rectangular magnetic structure with reduced cost, weight and size.